gst_hacks: Fix crash on Debian since upgrade to GStreamer 1.4

This fixes a double-free encountered on my Debian system since I upgraded
to GStreamer 1.4, although the potential for the issue has existed longer
than that.

Previously our description of `GstMemoryInfo` as a `ctypes.Structure` only
included the public members mentioned in the documentation, but the
structure in C is actually 32 bytes larger due to private fields and
padding.

We would allocate these structures in Python (via `ctypes`) and then pass
pointers to them to the GStreamer functions `gst_buffer_map` and
`gst_buffer_unmap`.  These functions now make use of these extra bytes but
because we weren't allocating a large enough buffer we were getting a
buffer-overflow causing a double-free to be reported and a crash.

This commit fixes this issue by copying the structure information from the
header file rather than the documentation, which is the true definition of
the structure.
pylint.sh: Silence uninteresting output with GStreamer 1.4

Since upgrading my Debian system to GStreamer 1.4 pylint tells me:

    /usr/lib/python2.7/inspect.py:65: Warning: /tmp/buildd/glib2.0-2.40.0/./gobject/gsignal.c:1631: parameter 1 of type '<invalid>' for signal "GstBus::sync_message" is not a value type
    /usr/lib/python2.7/inspect.py:65: Warning: /tmp/buildd/glib2.0-2.40.0/./gobject/gsignal.c:1631: parameter 1 of type '<invalid>' for signal "GstBus::message" is not a value type

Over and over again.  There doesn't seem to be any real problem so I
silence this warning in `pylint.sh` much like the other warnings.
GStreamer 1.4 support: Work around GStreamer overlay bug

Since upgrading my Debian system to GStreamer 1.4 the test
`test_ocr_on_live_video` has been failing with the error:

    FAIL: test.py: UITestError: Internal data flow error.: Internal data flow error.
    gstbasesrc.c(2933): gst_base_src_loop (): /GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0:
    streaming task paused, reason not-negotiated (-4)

The source pipeline in this test contains a `textoverlay` element.  This
has changed in GStreamer 1.4 to use the new `GstVideoOverlayComposition`
mechanism optionally allowing sinks to do any required compositing as this
may be more efficient.  This adds additional meta information to the
output caps if `textoverlay` thinks the downstream elements can handle
composition.

In this case downstream can't handle composition, but there is a
`decodebin` in the middle which obscures this from upstream so the meta
caps get added anyway.  When `decodebin` comes to create its output pad
it fails to link with the rest of the downstream elements because the caps
have the meta info and don't match.

This commit works around this bug by making `videotestsrc` output RGB
video that `textoverlay` can composite onto directly.  I have not yet
reported this bug upstream.

For more information see GStreamer bugzilla [#732040]

[#732040]: https://bugzilla.gnome.org/show_bug.cgi?id=732040
Merge pull request #210 from wmanley/gst-1.4

Fixes to work with GStreamer 1.4